"The box is filled with glass."
Translation:Kassen fyldes med glas.
"The box is filled with glass" is past tense, and the Danish translation should surely be "Kassen er fyldt med glas". Duo's translation "Kassen fyldes med glas" would be in English, "The box is being filled with glass".
Looks more like present passive to me (as in the box currently has glass inside it), "is being filled" is also a correct translation of the Danish, although I would maybe say "bliver fyldt" for this continuous sense, but I'm not sure if there is any difference between the two in Danish. The past tense would be "The box was filled with glass" (Kassen fyldtes med glas)